Why did Oberon feel compassion for Helena? What did he ask Puck to do to Demetrius?

In Act II, scene 1 of Shakespeare's comedy A Midsummer Night's Dream the fairy queen Oberon witnesses Demetrius's cruel rejection of Helena.
Demetrius once swore he loved Helena but has since transferred his affections to her friend Hermia, who has run away with her love Lysander. While Demetrius searches the woods, the realm of the fairies, for Hermia, Helena trails after him, basically begging him to love her back.
Oberon is invisible and witnesses the entirety of their fight. When his servant, the fairy Puck, returns from fetching the magic flower that Oberon sent him off to find, Oberon instructs him as follows:

Take thou some of it, and seek through this grove:A sweet Athenian lady is in loveWith a disdainful youth: anoint his eyes;But do it when the next thing he espiesMay be the lady: thou shalt know the manBy the Athenian garments he hath on.Effect it with some care, that he may proveMore fond on her than she upon her love:And look thou meet me ere the first cock crow.

Essentially, he is instructing Puck to cast a love spell on Demetrius to make him return Helena's affection. Oberon describes Helena as "sweet" and Demetrius as "disdainful," thus making it clear that Helena has won his sympathy. Note that the initial intention of this prank is not just to make Demetrius return Helena's love but actually to make him even "more fond" than she is of him, thus reversing the power structure of their relationship.